Swap toilet and sink...and then some

I bought a cape about 2 years ago, built in 1950. The second floor was finished in the 60's or 70's. The time has come for a little remodeling....

In the second floor bath (full) I want to rearrange the sink and toilet locations, the current layout is not very appealing. All the walls/ceilings have been stripped to studs (to reveal a lovely insulation that amounts to little more than corregated paper sandwiched with tinfoil ). These two fixtures are on the same wall, but seperated by the main vent/stack(cast) and a chase for same - sticks out about 20" from wall, about 8" wide. At the top of the stack is a connection for venting the tub/shower (copper). In the very near future, the roof will be replaced - very small leak in roof where stack exits thru roofline. Eventually, the bath on the first floor will also be gutted and remodeled ~ cosmetically - tiles and new fixtures, no real moving of locations.

Would it be best to remove the cast that is there and replace with ABS/PVC? or just use the sheilded fittings/adaptors to reconnect?
What are the clearances/layout for a new toilet (rough-in)?
What are the clearances/layouts for a sink (drian/supply) - the sink will have a base cabinet.